Preparation of melamine



Unitid te can Cyanamid Company, New York, N. Y., a corporation of Maine N Drawing. Application September 18, 1953, Serial No. 381,128

6 Claims. (31. 260-4495) The present invention concerns the preparation of cyanamide and certain of its polymers, 'and more 'particularly is directed to the preparation of melamine.

It has been found that when urea or biuret is heated in contact with an inert, inorganic heat-stable highly porous adsorbent of high surface area such as silica gel at a temperature of at least 220 C. a reaction. mass is formed containing melamine 'and/orucyanamide, from which these products may be recovered in various ways customary in the art. 1

This is a continuation-impart of .my co-pending application Serial No. 267,007, filed January 17,1952, now

abandoned.

The following examples illustrate the invention.

EXAMPLEI H U H Into a 1 liter Florence flask was charged 50;g. of silica gel.. This silica gel containedsevei'alpercent adsorbed water vapor, but did not prevent its catalytic operation. Powdered urea rriol, -1 5 g.) was then added to the silica gel and the two materials mixed byswirling the flask for a few minutes. The flask was heated ona sand bath, the surface temperature of which (as measured by a thermocouple) was approximately 340360 C. As the reaction mass was brought up to re ture, water vapor began to condense on the upper portion of the flask. The flask was left on the sand bath for minutes, after which it was removed and allowed to cool. The flask (and contents) was then washed with hot water and the slurry filtered hot. The filtrate was tested for cyanamide and melamine by the well-knownsilver nitrate and picrate methods respectively. A fair yield of both was obtained. A small amount of dicyandiamide was also found. 1 7

Instead of leaching the reaction mass with water alone, it can be first extracted with ether (in which oyanamide is soluble, but in which dicyandiamide and melamine are insoluble), followed by aqueous extraction to recover the 7 two latter compounds separate from cyanamide. ll he latter aqueous extraction is made hot in the known way, and the filtrate allowed to cool, whereupon melamine crystalizes from solution. The residual solution containing dicyandiamide can then be concentrated and evaporated to recover its dicyandiamide.

,Melamine can also be recovered from the silica gel reaction mass by other means w'ell knownre those Lskilljed in the art. 7 A preferred method is to sublime "it off the gel'with hot ammonia.

EXAMPLE 2 69 grams of silica gel "were heated. overnightin a inufile furnace at 600. C. to remove surface moisture whereby the silica lost approximatelyv4%fin 'weighb'providing 66 grams of material. Thissilica was mixed 33 grams of urea and the mixture was placed in a vertical reaction column inches high by A" in diameter.

. 2,760,?! Patented Aug. 28, 1956 "ice The mixture was supported approximately /3 of the way down in the column, where it occupied a vertical space about 20" long. The column was equipped at the top with an ammonia inlet port and with a vapor exit at the bottom discharging into a melamine collection vessel maintained at about C. The collection vessel caused melamine vapor to deposit therein as solid while simultaneously permitting byproduct carbon dioxide together with ammonia to pass through without solidification as ammonium carbamate. At the beginning of the reaction the ammonia inlet port was opened so that ammonia vapor swept slowly through the reaction mixture at the rate of about one liter of ammonia per minute measured at room temperature. At the same time the vertical reaction tube, which was heated by means of electrical resistance, was brought to reaction temperature as rapidly as possible. Under the conditions given in this example approximately O.751.0 hour was required to bring the mixture to the operating temperature of 275- 375 C. The temperature was measured by means of a thermocouple well in the center of the reaction charge. The first trace of melamine appeared in the collection chamber when the thermocouple registered about 190 C., and increased in recovery per unit time as the converter became hotter. in a 24-hour reaction period, the bulk of the 8g. of melamine was collected in the receiver. The silica was then removed, mixed with 40 g. urea, and heated in ammonia for another 5 hours, yieldingan additional 12.2 g. of melamine. The cycle was repeated a number of times, as shown in the following table.

T able I [Consecutive additions of urea to same silica gel catalystififi g., 58-60 mesh). Reaction temperature 275-375 C.NH 1 liter/minute] a Urea, Gumu- Mela- Cumu- Day Clock Time added, lative mine lative g. urea, g. out, g. melamine, g.

12:00 N 33 33 12:00 N- 0 33 8 8 12:0 40 73 5 P. '0 73 12.2 20. 2 9 A. 0 73 2. 8 23.0 9 A. 42 v 1 P. 0 115 10.1 33.1 1 P. 25 4 P. 0 140 6. 59 39. 6 5 "P. 0 140 1.55 41. 2 9A. 43 183 1 P. 0 183 13.7 54. 9 v1 P. 25 208 4 P. 0 208 9. 85 64. 8 9 A. 0 208 5. 0 69. 8 9 A. 42 250 1 P. 0 250 11 5 81. 3 1 P. 25 275 4:30 0 275 6. 65 88. 0 9 A. 0 275 5. 2 93. 2 9 A. 44 319 J... i 1 P. i 0' 319 11.0 104. 2 11?. 31 350 .......e 4:30 0 350 10.0 114. 2 9 A. O 350 2. 7 116.9 5 9 A- (0 (0. 9 A. I 0 350 4. 0 120. 9

1 Turned ofi heatand NH;. 1 Turned on heat and N 11:.

EXAMPLE 3 The apparatus consisted of a l-lit'er fiat-bottom, 4-

necked stainless steelflask. It was equipped with a cen- 7 0 was fitted with a liquid urea feed and the fourth with a at 100 C. to condense vapor melamine to solid melamine and to pass the residual NHs and CO2 to an ammonium carbamate recovery unit.

was then removed, mixed with another 50 g. urea, and heated as before giving 117 g. A third treatment gave 143 g. of mixture, which analysis showed to contain much ammeline and small amounts of melamine and The melamine synthesis flask was heated in an electric 5 cyanamide. To this 143 g. was added anoth r gjacket to 400 C., while continuously stirring at about portion of urea, s IniXtuIfi 8 heated 50 R. P. M., and the NI-ls and urea feeds opened. for 4% hours in the same way, during which time 56 g.

The urea feed rate was maintained at about 45 g./hr. of melamine sublimed oil. The rate of melamine reand NH3 at about 3 liters/minute for hours. Melamine covery was apparently in excess of 0.25 g./hr./g. urea recovered was 12.6 g. 10 for several minutes when the reaction temperature passed through the range 340350 C., 24 g. being distilled EXAMPLE 4 out in minutes in this range, but thereafter the rate fell,

Using substantially the same apparatus and procedure 6 g. b ing colle ted in the succeeding 15 minutes. as in Example 3, 10 g. of biuret was fed through a solids addition tube to 50 g. of silica gel over a period of about 15 EXAMPLE 17 1 hour, and nearly 4 g. of melamine was condensed from in this run urea was continuously converted to melathe effluent. mine in a fluidized silica gel bed. The fluidized column comprised a stainless steel tube A1 in diameter by 60 EXAMPLE 5 in height. 250 g. of silica gel of the following screen The reactor used in this run comprised a heat resistant 20 analysis was placed in the column: glass tube 24 inches long by 1 inch in diameter, heated at about 350 C. by means of resistance wire. A mixture Mesh range Percent slhca 60-100 15 of urea and silica gel was fed into one end of the tube 100200 35 from an overhead hopper and pushed slowly through the 200 325 35 tube by means of a screw conveyor. Ammonia at the 25 325 15 rate of 1 liter per minute was passed co-cunently through the advancing charge. The vapor efiluent was passed This amount of silica gel occupied 36" of vertical space through a melamine condensing flask maintained at 100 in the column in the position of rest, and 60" when fluid- C., in which melamine condensed on the walls. As the ized with a stream of ammonia passed into the bed at the initial charge, 300 g. of silica gel, the bulk of which was rate of 2.5 liters per minute. The column was heated to 2860 mesh, was mixed with 200 g. of urea, and this a temperature of 350 C. by means of electrical resistance mixture passed through the reactor over a period of eight heaters coiled about the column. The column was hours, during which 55 g. of melamine was collected. thermally insulated to prevent undue heat loss. The urea The residual catalyst was allowed to fall into a collectfeed mechanism comprised a screw conveyor to permit ing flask underneath the terminal end of the furnace. At the steady excrusion of powdered urea into the base of the end of the first run it was withdrawn, admixed with the column where it could be swept up into the fluidized 100 g. of urea, and the mixture passed through the rebed by the incoming ammonia. As soon as the column actor under the same conditions as before, providing attained operating temperature the introduction of urea an additional 27 g. of melamine. was begun. 15 g. of urea was gradually fed into the The following table summarizes experiments conducted reactor over a period of 60 minutes. The rate of feed in substantially the same apparatus and manner of Exwas thereafter increased to 21 g. per hour within the next ample 2. hour, after which time this feed rate was continued for an Reactant, g. Sweep gas, Time in Melliter/min. heating amine Dry Reaction Time at from room con- Rnn silica temp, reaction temp. to densed No. gel, g. O. temp. reaction from Urea Biuret NH; Other temp., vapor min. efiiuent,

60 1 1 min. e0 50 1 60 so 60 1.8 60 60 1 e0 40 1 115 5s 39 0. 6-0. 7 about 60... 11. 4

60 50 1 4 hrs do 14 60 30 5 l 1% hrs do 5 16 60 52.5 1 5 about 40... 18

1 Contained ammonium carbamate; no melamine in effluent but gel contained a little melamine. 2 Gel residue contained much ammeline with a small amount of melamine.

l Contained 1 g. melamine. 4 Contained melamine.

* The NH was bubbled thru 20 ml. H2O at r. t. and the resultant vapor mixture used as a sweep gas. 5 Recovered as aqueous slurry by condensing some of the H20 in efliuent along with the melamine.

It is preferred to load the silica with as much melamine-forming material as possible, during either batch or continuous operations. The following is an example of loading the silica.

EXAMPLE 16 A mixture of 60 g. silica gel and 50 g. urea was heated in the apparatus of Example 2. feeding NI-Ia at 1 liter/minute, until the temperature reached about 210 additional 26.5 hours. The efliuent gases, comprising melamine vapor, ammonia, and carbon dioxide were led into a vessel maintained at about 100 C. to condense melamine without condensing ammonium carbamate. The melamine so recovered was extremely pure, having passed directly from the vapor phase to the solid phase. The total melamine recovered during the entire operation amounted to 157 g., 80% of theory.

The following table summarized further experiments C. (30 minutes required). The resultant mixture (88 g.) conducted in the apparatus and manner of Example 17.

grease:

7 Table 3 V Ex. 18 Ex. 19 Ex. 20 Ex. 21 Ex. 22

Urea, g 1 50 2 90 80 133 5 180 Silica gel, g 1 200 I 190 3 210 200 125 NH; rate, L./min..; 1 ,4 4 v4 5 '3 Reaction temp., C 320-350 350 300-350 250 .400 Time at reaction temp., hr 5. 5 16 5. 5 v 7 9 Melamine condensed from efiiuent, g 17 '29 29 31 58 Yield, percent of theory 97 92 67 96.6

1 The urea-silica mixture was placed in the column at room temperature and heated to reaction temperature with continuous fluidization by the incoming NHs. f

' Mixture heated to 80 'C. before placing in fiuidizing'colunm.

3 Tothe silica catalyst remaining from Example v19 was added suflicient clean dry silica gel to bring the total weight to 210" g. The catalyst was then placed in the column. andheated to reaction temperature with continuous fluidization by means of the incoming N H3. The urea (fine powder) was blown into the hot catalyst gradually over the 5% hour reaction period by charging it to the fiuidizing NHz. 1

4 The yield includes a little melamine or the equivalent remaining on the silica from Example 19. The total yield from both Example 19 and Example 20, 58 g., was 98% of theory.

5 Fed to fluidizing NH gradually over the reaction period.

The operation of the process is not confined to atmospheric pressure conditions. The reaction apparently proceeds independently of pressure, and can be conducted under subor super=atmospheric pressure and still obtain advantages from the 'use' of silica gel. The following table summarizes runs at super-atmospheric pressure ranging up to about 360 pounds per square inch gauge in a pressure-resistant stainless steel fiuidizing column similar to that used in Examples 17-22. The urea was fed gradually over the reaction period as an atomized liquid at about 135 C. into the lower region of the fluidized column of silica gel.

Table 4 1 Recycled vapor efliuent from which the melamine had been stripped was used as the fiuidiziug medium; it consisted of a mixture of 2 moles NH: per mole of 002 with traces of melamine.

"A portion of vapor efliuent from which melamine had not been stripped was recycled as the fluidizing medium. It consisted of about 6NH3/3C02/1 melamine.

THE ADSORBENT It has been found that virtually any inert adsorpti've material of high internal surface area and/or volume can be adapted to serve as a catalyst in the conversion of urea and biuret to cyanamide and its polymers. The action appears to be predominantly physical, and is apparently induced best by materials of high surface area in which a considerable amount of the surface is actually in the form of the walls of innumerable submicroscopic capillaries. The activated oxide gels of the amphoteric elements qualify admirably in this respect. For example, it is well known how to prepare gels of silica and alumina such that the internal volume of a given particle is considerably more than half the apparent volume of the particle. The same techniques have been widely utilized in the past for preparing activated gels of titania, zirconia, thoria, and the like. There are also many naturallyoccurring absorbents available commercially in activated form that have been found suitable. These include pumice, diatomitc, infusorial earth, and the like, which inherently possess the desired amorphous silica structure and can therefore be activated by reduction of the water content. Others, such as the clays and clay-like materials, e. g., kaolin, bentonite, bauxite, and fullers earth, are customarily activated by an acid wash prior to thermal dehydration, thereby dpdsitingin sim on ea n'flari' r particle of material a layer of silica and/ or alumina gel.

The structures of gels of silica, silica-alumina and the like can be varied by various pro-treatments to give materials having predominantly "pores of a large 'sii or any intermediate diameter from 60 to 70 A. to ve'r'y 'low values. Also, the gels in question can ays surface areas covering the range of -650infi/g. Gels having specific surfaces of 600 m. /g. and pore volur'n's' jQf 09 cc./g. have been found satisfactory as have g'els' having surfaces of 450 m. g. and pore volumes of 0.2j6fc'cJg. These are not to be construed as limits of workability of the gel but only as examples of the variations instrucmre possible. I a 7 As normally prepared, the activated catalysts maycar'ry 220% by weight of adsorbed water. It is neither'necessary nor desirable to remove 'all of this water (as by preliminary strong heating). In fact, heating-'silica 'gl overnight in a muflle furnace at 600 C. will ordinarily remove only /2- /3 of its adsorbed water. However, whatever water is not removed by preliminary heating is eventually nearly all removed by hydroly'tic reactions with reactants on the silica, yielding NH: andCOz.

While silica gel is used in most of the examples in this s'pecification, it is to be understood that this -is but representative of the general class of inert, inorganic, heatstable, porous, adsorbent materials described as suitable in this section. I

In all of the above runs biuret can be substituted for urea in amounts proportionate to the following equations:

6 urea- 1 melamine+6NH3+3CO2 3 biuret l melamine+3NH3+3CO2 Either reactantmay be heated with silica gel or the like at temperatures within the range 220-600 C. to form mixtures of cyanamide and melamine. In the range 220-400 C. the product is mainly melamine. Above 400 C. the melamine begins to decompose to cyanamide and other products, e. g.,'melam, etc. It is very difficult to remove cyanamide from silica gel by strong heating, some remaining even at red heat.

The preferred temperature range for making melamine by this process is about 250400 C., and more preferably, 275 350 C.

EXAMPLE 26 A mixture consisting of 20 g. urea and 40 g. silica gel predried at 400 C. was heated at 250 C. for 15 minutes in a 300 cc. titanium-lined autoclave under the autogenously produced pressure of about 300 p. s. i. The yield of melamine was 69.5%.

EXAMPLE 27 The above experiment was repeated, using silica gel predried at 650 C. The yield of melamine was 83.8%..

EXAMPLE 28 The experiment of Example 26 was repeated, using silica gel predried at 850 C. The melamine yield was EXAMPLE 29 The experiment of Example 26 was repeated, using diatomaceous earth predried at 650 C. as the absorbent. The yield of melamine was 10%.

EXAMPLE 30 A mixture of 20 g. urea and 60 g. of prcdried silica was heated in a 300 cc. autoclave open to the atmosphere at 300 C. for 1 hour, giving a reaction mass containing melamine in 48% yield. A similar 2-hour run gave a 73% yield.

In one modification of the invention, the urea-silica gel mixture is heated in the absence of any sweeping gas in an initial melamine-forming vessel optionally equipped with vapor exit lines for the collection of ammonia and carbon dioxide evolved. The thus-formed melamine can then be sublimed from the silica gel in a second operation.

I claim:

1. The method of preparing a reaction mass containing melamine which comprises bringing a reactant selected fromthe group consisting of urea and biuret into reactive contact with a silica gel under substantially atmospheric pressure and at a temperature of from about 220 to about 400 C.

2. The method which comprises combining a silica gel and urea, the urea being initially in the solid phase, into an intimate mixture and heating the admixture under substantially atmospheric pressure and at a temperature of from about 220 to about 400 C., for sufiicient time to form a reaction mass comprising melamine.

3. The method which comprises combining a silica gel and urea, the urea being initially in the solid phase, into an intimate mixture and heating the admixture in the presence of added ammonia under substantially atmospheric pressure and at a temperature of from 220 to about 400 C., for sufiicient time to form a reaction mass comprising melamine.

4. The method which comprises combining a silica gel and biuret, the biuret being initially in the solid phase, into an intimate mixture and heating the admixture under substantially atmospheric pressure and at a temperature of from about 220 to about 400 C., for sufiicient time to form a reaction mass comprising melamine.

5. The method which comprises combining a silica gel and biuret, the biuret being initially in the solid phase, into an intimate mixture and heating the admixture in the presence of added ammonia under substantially atmospheric pressure and at a temperature of from about 220 to about 400 C., for sufficient time to form a reaction mass comprising melamine.

6. The method of preparing a reaction mass containing melamine which comprises bringing a reactant selected from the group consisting of urea and biuret into reactive contact with a silica gel and added ammonia under substantially atmospheric pressure and at a temperature of from about 220 to about 400 C.

References Cited in the file of this patent UNITED STATES PATENTS 2,500,489 Dyer Mar. 14, 1950 2,524,018 7 Mackay Sept. 26, 1950 2,566,231 Paden Aug. 28, 1951 

1. THE METHOD OF PREPARING A REACTION MASS CONTAINING MELAMINE WHICH COMPRISES BRINGING A REACTANT SELECTED FROM THE GROUP CONSISTING OF UREA AND BIURET INTO REACTIVE CONTACT WITH A SILICA GEL UNDER SUBSTANTIALLY ATMOSPHERIC PRESSURE AND AT A TEMPERATURE OF FROM ABOUT 220* TO ABOUT 400* C. 